The invention relates to a high-pressure discharge lamp provided with a discharge vessel with a ceramic wall which is closed at an end by a projecting plug through which a lead-through construction extends from an end of the projecting plug to an electrode arranged in the discharge vessel, part of said lead-through construction being a cermet.
A lamp of the kind mentioned in the opening paragraph is known from WO 96/28839. The known lamp is a metal halide lamp. Such lamps are widely used in practice and have a high luminous efficacy combined with favorable color properties. The discharge vessel of the lamp contains one or several metal halides in addition to Hg. The lead-through construction in the known lamp achieves a gastight connection to the projecting plug by means of a melting glass fusion joint. The gastight connection is realized to a major extent between the end of the projecting plug and an electrical conductor connected to the cermet and extending from the end to the exterior. Nb is used as a conductor in the area of the gastight melting glass fusion joint in practice because this has a coefficient of expansion which differs only very slightly from that of the ceramic wall of the projecting plug. A further advantage of Nb is its ductility property. Nb, however, is not resistant to halides. To counteract attacks on the Nb, the melting glass fusion joint extends along part of the cermet in the known lead-through construction. To obtain a continuation length of the melting glass fusion joint along the cermet which was found to be desirable, the projecting plug is made impermeable to light over part of its outer side. It is achieved thereby that the electrical conductor extending from the cermet into the projecting plug is not in direct contact with filling ingredients inside the discharge vessel.
A ceramic wall in the present description and claims is understood to be a wall made from one of the following materials: monocrystalline metal oxide (for example sapphire), densely sintered polycrystalline metal oxide (for example Al2O3, YAG), and densely sintered polycrystalline metal nitride (for example AlN).
It was found to be a disadvantage of the known lamp that the melting glass fusion joint itself appears to be vulnerable to attacks by filling ingredients from the discharge vessel during lamp life. This gives rise to leaks in the course of time, which results in the end of lamp life.
The invention has for its object to provide a means for counteracting the above disadvantage. According to the invention, a lamp of the kind mentioned in the opening paragraph is for this purpose characterized in that the cermet is directly fastened to the projecting plug by means of a sintered joint.
It is an advantage of the lamp according to the invention that the use of melting glass can be dispensed with. It is indeed possible to form the sintered joint between the projecting plug and the cermet into a hermetically closed seal by means of a suitable sintering process which is known per se. A further advantage is that the projecting plug need not be made impermeable to light over part of its outer surface. This means a simplification in lamp manufacture, which is a considerable improvement in mass production on an industrial scale. Moreover, the electrical conductor no longer forms a necessary part of the lead-through construction, which offers a greater freedom of choice as regards the materials of this conductor.
In an advantageous embodiment of the lamp according to the invention, the projecting plug extends over a length L, and the sintered joint has a length of at most 0.8L. The inventors have found that the risk of cracks and fractures occurring in the projecting plug is very strongly reduced in this manner. This relates in particular to the risk of the formation of cracks and fractures owing to thermal stresses during the sintering process. A further improvement can be realized here if the sintered joint extends into the projecting plug in a direction from the end to a distance away from the end of at most 0.5 mm. In a further advantageous embodiment of the lamp according to the invention, the cermet has a tapering shape adjacent the end and is provided with a narrowed portion. This renders it possible to have the cermet extend to outside the projecting plug with its narrowed portion, while retaining the favorable properties of the sintered joint of limited length. A cermet extending to outside the projecting plug is advantageous for an efficient large-scale mass production of lamps, because a simpler fastening of the electrical conductor is possible, whereby the risk of production wastage is further reduced.
The invention is favorable in particular for lamps having a comparatively high power rating, for example of 100 W or more.